In a closed-loop emisson control system for internal combustion engines, the concenration fo of exhaust composition is detected to represent the mixture ratio of air-to-fuel combusted in the cylinder in a given cycle in order to control the mixture ratio in a subsequent cylinder cycle in such manner that the difference between the sensed ratio and a desired value is compensated. Conventionally, residual oxygen is sensed by a zirconium type oxygen sensor which provides an output having a sharp characteristic change in the vicinity of the stoichiometric air-fuel ratio. However, the conventional oxygen sensor is limited in performance by the temperature of the exhaust emissions. Therefore, it is sometimes the case that the output of the sensor remains at a low voltage level during the start of the engine or when fuel supply is cut off upon sudden deceleration of the engine. A similar situation occurs when the sensor should fail. With the sensor output being at the low or zero voltage level, the control signal would be such that an extremely rich mixture is supplied to the engine irrespective of the engine operating conditions.
A prior art emissiion control system utilizes the fluctuation of the sensed oxygen concentration due to the inherent transport delay time of the engine as a signal for determining the performance of the exhaust sensor. If the fluctuation ceases, a detector senses the absence of the fluctuation to clamp the air-fuel ratio to a constant value to prevent the mixture from becoming too rich or too lean. Although such clamping action is advantageous when the sensor should fail, it is disadvantageous during sudden deceleration and particularly when the engine is restarted after warm-up operation. Upon restart of the engine after warm-up operation, the mixture is leaned and the output of the exhaust gas sensor remains low so that the detecting circuit permits the clamping circuit to operate to cause the engine to run under an open loop control mode. Therefore, as long as the lean mixture condition exists the closed control operation is disabled generating noxious exhaust components to the atmosphere even though the gas sensor is working properly.